In modern semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such devices. Conventionally, a flip-chip attachment method has been used in the packaging of IC chips. In the flip-chip attachment method, instead of attaching an IC die to a lead frame in a package, an array of solder balls is formed on the surface of the die. The formation of the solder balls is normally carried out by an evaporation method of lead and tin through a mask for producing the desired solder balls. More recently, the technique of electro-deposition has been used to produce solder balls in flip-chip packaging.
Other solder ball formation techniques that are capable of solder-bumping a variety of substrates have also been proposed. These techniques work fairly well in bumping semiconductor substrates that contain solder structures over a minimum size. One of the more popularly used techniques is a solder paste screening technique which can be used to cover the entire area of an 8 inch wafer. However, with the recent trend in the miniaturization of device dimensions and the reduction in bump-to-bump spacing (or pitch), the solder paste screening technique becomes impractical. For instance, one of the problems in applying solder paste screening technique to modem IC devices is the paste composition itself. A paste is generally composed of a flux and solder alloy particles. The consistency and uniformity of the solder paste composition become more difficult to control with a decreasing solder bump volume. A possible solution for this problem is the utilization of solder pastes that contain extremely small and uniform solder particles. However, this can only be done at a high cost penalty. Another problem in using the solder paste screening technique in modern high density devices is the reduced pitch between bumps. Since there is a large reduction in volume from a screened paste to the resulting solder bump, the screen holes must be significantly larger in diameter than the final bumps. The stringent dimensional control of the bumps makes the solder paste screening technique impractical for applications in high density devices.
Another concern with the solder paste screening technique is that the solder paste is normally applied directly to substrates through a screen mask which contains holes aligned to the paste-receiving pads on the substrate. The substrate may be a silicon wafer or a substrate of any other material. Since the paste is applied directly by the screening method, any problems occurring during the process result in rework of the substrate.
A more recently developed injection molded solder (IMS) technique attempted to solve these problems by dispensing molten solder instead of solder paste. However, problems have been observed when the technique is implemented to wafer-sized substrates. U.S. Pat. No. 5,244,143, assigned to the common assignee of the present invention, discloses the injection molded solder technique and is hereby incorporated by reference in its entirety. One of the advantages of the IMS technique is that there is very little volume change between the molten solder and the resulting solder bump. The IMS technique teaches the use of a two inch wide head that fills boro-silicate glass molds that are wide enough to cover most single chip modules. A narrow wiper provided behind the solder slot passes the filled holes once to remove excess solder.
The IMS method for solder bonding can be carried out by typically applying a molten solder to a substrate in a transfer process. When smaller substrates, i.e., chip scale or single chip modules (SCM's) are encountered, the transfer step is readily accomplished since the solder-filled mold and substrate are relatively small in area and thus can be easily aligned and joined in a number of configurations. For instance, the process of split-optic alignment is frequently used in joining chips to substrates. The same process may also be used to join a chip-scale IMS mold to a substrate (chip) which will be bumped. Over a small area, flux material which is dispensed in a thin layer over the chip prior to being joined to a solder-filled IMS mold keeps both surfaces in intimate contact due to surface tension of the liquid flux. This is desirable such that when solder in the mold balls-up upon heating to a melting temperature in a furnace, it contacts the solder-receiving pad which is normally covered with gold or other solder-wetting alloy.
Another method that does not have the limitations of the solder paste screening technique of significant volume reductions between the initial paste and the final solder volume is the molten solder screening (MSS) method. In the MSS method, pure molten solder is dispensed. When the MSS solder-bumping method is used on large substrates such as 8 inch or 12 inch wafers, surface tension alone is insufficient to maintain intimate contact between a mold and a substrate. In order to facilitate the required abutting contact over large surface areas, a new method and apparatus for maintaining such are therefore necessary.
It is therefore an object of the present invention to provide an apparatus for transferring solder bumps from a mold to a substrate that does not have the drawbacks and shortcomings of a conventional apparatus.
It is another object of the present invention to provide an apparatus for transferring solder bumps from a mold to a substrate that is constructed by a base member, a compressible member and a lid member.
It is a further object of the present invention to provide an apparatus for transferring solder bumps from a solder mold to a solder-receiving substrate by utilizing a transfer fixture which is capable of applying uniform pressure on a mold/substrate assembly during the transfer process.
It is another further object of the present invention to provide an apparatus for transferring solder bumps from a solder mold to a solder-receiving substrate which includes a transfer fixture for applying an uniform pressure on a mold/substrate assembly and allowing lateral movement of the assembly relative to the fixture due to differences in the coefficients of thermal expansion of the various components.
It is still another object of the present invention to provide an apparatus for transferring solder bumps from a solder mold to a solder-receiving substrate which includes a solder mold constructed of a substantially transparent material so that the mold cavities contained therein can be readily examined.
It is yet another object of the present invention to provide an apparatus for transferring solder bumps from a solder mold to a solder-receiving substrate which includes a lid member having a skeletal configuration such that the mold/substrate assembly can be visually inspected with the lid member mounted on the assembly.
It is still another further object of the present invention to provide an apparatus for transferring solder bumps from a solder mold to a solder-receiving substrate by using a lid member equipped with a plurality of compression pins having contact points for applying a substantially uniform pressure on a mold/substrate assembly while allowing lateral movement of the assembly due to mismatched coefficients of thermal expansion of the various components during a thermal cycle.
It is yet another further object of the present invention to provide an apparatus for transferring solder bumps from a solder mold to a solder-receiving substrate which includes a cellulosic foam member positioned between a mold/substrate assembly and a base member for allowing lateral movement of the assembly relative to the base member due to differences in the coefficients of thermal expansion.